In radiation detector systems, cooling of the radiation detector and input components of the amplification circuit generally reduces electronic noise and enhances spectroscopic performance of the system. Liquid nitrogen (LN2) has traditionally been used for such cooling purposes.
A major weakness of LN2 cooling is the necessity for a large reservoir (i.e., dewar). Also, using LN2 is hazardous, inconvenient, and pricey. By way of example, severe burns can result from skin contact with LN2. In addition, the LN2 dewars must be refilled on a regular basis (e.g., twice per week) in order to maintain the detector at operating temperature.
Further, radiation detectors are frequently mounted at locations that are not easily accessible, and refilling the LN2 dewar is often an unsafe and uncomfortable operation, during which a user must carefully follow safety procedures or risks injury. The cost of purchasing, storing and handling LN2 over the lifetime of a radiation detector can be very high. Since LN2 evaporates from both the detector dewar and the storage tank, a waste cannot be prevented. Also, during the refilling process, significant amounts of LN2 are typically lost due to evaporation and spillage. Thus, in addition to the expense of LN2 consumed for cooling the detector, additional expenses are incurred because of LN2 loss during dewar refilling and storage.
By way of example, about ten liters of LN2 per week evaporate from a standard detector dewar, and about the same amount is lost during transfer and through evaporation from the storage tank. Therefore, a standard 160-liter storage tank lasts about eight weeks and must be refilled an average of 6.5 times per year. Hence, the annual cost of cooling an LN2-based detector for operation is approximately $2,500 per year. To the annual cost of LN2 must be added the cost of labor for handling LN2 and refilling the detector dewar. Even if this labor averages only twenty minutes per week, it can add hundreds of dollars per year to the cost of servicing an LN2-cooled detector.
In addition, the handling of LN2 often requires the purchase of a special liability insurance. Further, conventional LN2-cooled radiation detectors are bulky, due to their large capacity (e.g., nine liters) dewars, and typically weigh over forty pounds. Placed at the end of a long support structure, an LN2-cooled detector is cantilevered, and is vulnerable to vibrations that interfere with high resolution imaging in its applications in electron microscopes.
Radiation detectors utilizing thermo electrical cooling (TEC) (i.e., Peltier cooling) are being frequently used in many applications. The development of silicon structures such as low leakage current p-i-n and drift type detectors as well as high resistivity compound semiconductor detectors (HgI2, CdTe, CdZnTe and others) allow for construction of high energy resolution, spectroscopy systems without requiring LN2 cooling. Also, attempts are being made to develop Si[Li] systems that can operate with TEC cooling.
The TEC has distinctive advantages over the use of LN2. A TEC is a small heat pump that has no moving parts and can be used in various applications where space is limited and reliability is very important. The TEC operates using direct current by moving heat from one side of the module to the other with current flow and principles of thermodynamics. The theories behind the operation of TEC can be traced back to the early 1800's when Jean Peltier discovered that there is a heating or cooling effect when electric current passes through two conductors.
A typical single stage TEC includes two ceramic plates with p-type and n-type semiconductor material (e.g., bismuth telluride) between the plates. The elements of the semiconductor material are connected electrically in series and thermally in parallel. When a positive voltage is applied to the n-type thermo-element, electrons pass from the p-type thermo-element to the n-type thermo-element and cold side temperature decreases as heat is absorbed. The heat absorption (cooling) is proportional to the current and the number of semiconductor element pairs. The heat is transferred to the hot side of the cooler, where it is dissipated into a heat sink and/or the surrounding environment.
Although the amount of TEC cooling is proportional to the current applied, the power dissipated by Joule heating in the TEC is proportional to the square of the current, and it can be shown that half of this Joule heat must be pumped from the cold junction. For this reason, an increase in current above a certain level will result in less net cooling because the Joule heating is increasing at a faster rate than the cooling.
A multi-stage TEC is essentially two or more single stage TECs stacked vertically with fewer thermo-elements in each ascending stage. The multi-stage TECs are used to achieve larger temperature differential between cold and hot sides.
Despite its many advantages, the TEC is inferior to LN2 when it comes to heat pumping capacities and the lowest achievable temperature. However, in many cases use of the thermo electrical cooling is sufficient to achieve the desired performance characteristics of a spectroscopy system. Nevertheless, the above listed deficiencies typically impose strict limitations on the design of the spectrometer in terms of the placement of the cold components and dissipation of the heat generated by the TEC. In particular, it is difficult to extract heat from spectrometers having a configuration including a long probe design.
The long probe designs are often used in applications such as electron microscopy and many other x-ray fluorescence measurements. The detector needs to be placed in proximity to the samples being examined in order to assure a large solid angle of measurement. Very often the access to the sample is limited, for example, by electron focusing lenses or/and other instruments and objects including the sample holder.
For this reason, a front-end of the spectrometer is constructed as a long cylinder with a small diameter and the detector is placed at the front of the cylinder behind an entrance window. If the TEC is situated in proximity to the detector, there is a long path for the heat from the TEC to dissipate to the surrounding environment. A copper rod is typically used to conduct heat to an external radiator or a radiator and fan. Despite the fact that copper provides a good heat conductivity, there usually is a large temperature gradient generated along the heat conductor, thus elevating temperature at the hot side of the TEC.
By way of example, the temperature gradient generated due to 5 W heat energy conducting along a copper rod having 1.2 cm diameter and 15 cm length is 17° C. This phenomenon further limits the lowest achievable temperature at the cold side (i.e., the end thermally coupled to the radiation detector) of the TEC. In order to overcome this problem, other designs involve a water or coolant circulation at the base plate which is thermally coupled to the hot side of the TEC. This solution itself brings additional complexity and cost to the system. Moreover, the coolant flow may introduce vibration and noise to the spectrometric system.
An example of using a copper rod to conduct heat from the detector to the cold side of the TEC and then attaching the hot side to a copper base cooled by flowing water is described in U.S. Pat. No. 5,075,555 to Woldseth et al. Some of the other concerns associated with this method are as follow. First, the long copper rod (“cold finger”) causes a big loss in the cooling efficiency due to thermal radiation from the cold copper rod to the room temperate wall surfaces. The multi-stage TEC is a very low efficiency device, and losing temperature due to surface thermal radiation further complicates the design of an additional radiation shield surrounding the whole length of the copper rod (in some microanalysis products, a length of 10 inches (25.4 cm) or more). This radiation shield tube also needs to be cooled down by the same TEC.
Further, the TEC top stage is very fragile, and connecting it to the copper rod, even through a copper braid (“flexi cold finger”), can frequently cause the top stage of the TEC to break down as a result of mechanical shocks. In addition, cooling down the hot side of the TEC with flowing water is a relatively expensive solution. A chiller or a water radiator needs to be added, and in addition to the extra cost, it is also an additional bulky device which is inconvenient to add in many applications.
Therefore, a cooling system for a radiation detector system that overcomes one or more of the problems associated with above-mentioned LN2 cooling or TEC/copper rod cooling is desired.